1. Field of the invention
The present invention concerns a heat recovery method and device suitable for combined cycles.
It is more particularly concerned with a heat recovery method suitable for combined gas turbine/steam turbine cycles. The method utilizes a waste heat boiler at the exhaust of a gas turbine. The water in the boiler is first treated in a degassing unit connected to a first evaporator.
2. Description of the prior art
Combined cycles using gas turbines and steam turbines are a known method of producing electricity from natural gas or petroleum products. The steam cycle used is normally of the sub-critical type (at 110 bars) and comprises two or three pressure levels (110 bars, 28 bars, 4.6 bars) for increased overall thermal efficiency. Recent increases in gas turbine size and gas turbine exhaust temperatures also enable the steam cycles to be enhanced by introducing a reheater (approximately 540.degree. C.). The raw overall thermal efficiency of the cycle is thereby increased from 52.1 to 55.2%. However, the need to reduce emission of carbon dioxide and nitrogen oxides is encouraging further research into improving cycle overall thermal efficiency which is still deemed to be insufficient, especially under part-load conditions.
It is also known to use super critical steam cycles with only one pressure level, burning powdered coal to produce electricity with a high overall thermal efficiency (42 to 44%). However, further development of this technique is impeded by the problem of high-temperature corrosion of the terminal exchangers by sulfur and chlorine containing substances. An additional problem is caused by the mechanical strength of the screen materials. The addition of a flue gas treatment plant to reduce the emission of nitrogen and sulfur oxides significantly complicates operation and significantly increases the investment cost.
The circulating fluidized bed combustion technique can now be substituted for the combustion of powdered coal in a range of electrical powers below 300 MW as it can substantially reduce in situ the emission of oxides without requiring any additional processing of the flue gas.
The presence of fluidized bed exchangers, also called external beds, the principle of which is described in French patent FR-2 353 332, enables installation of high-temperature exchangers in a non-corrosive gas/solids environment with extremely high exchange coefficients and lends itself particularly well to a supercritical cycle configuration. Supercritical cycle size. For economic reasons associated with the additional investment costs, increasing the size of circulating fluidized bed boilers leads to a relatively much greater contribution of these fluidized bed exchangers to the thermal exchanges in the boiler loop. Therefore, these exchangers then constitute the only item which can be extrapolated, which is extremely favorable. Although highly advantageous from the economic and operating points of view, the overall thermal efficiency of this circulating fluidized bed combustion technique is limited. The efficiency is limited by the use of steam turbines with inlet pressures and temperatures currently around 275 bars and 565.degree. C. for live steam, producing an overall thermal efficiency of 42 to 44%. Also, part-load overall thermal efficiency is deemed to be insufficient as, in this case, the boiler operates under subcritical conditions.
It may appear advantageous to combine a gas turbine and a circulating fluidized bed at the level of their water steam cycles to increase the overall thermal efficiency of the overall cycle. However, the problem of using known plants such as a boiler on the exhaust side of a gas turbine with three subcritical pressure levels and a circulating fluidized bed boiler with one supercritical pressure level is complex to solve and to optimize. It would obviously be possible to transfer exchanges by, for example, superheating the intermediate pressure gas turbine steam in the circulating fluidized bed but this is not the optimum arrangement from the overall thermal efficiency point of view.
It would also be possible to effect the recovery of heat on the exhaust side of the gas turbine in supercritical mode with one pressure level but this has the drawback of degraded overall thermal efficiency under part-load conditions because of the resulting subcritical operation with one pressure level.
An additional problem may be caused by the further presence of a waste heat boiler on the output side of a gasifier used to convert a solid fuel into gas. The solid fuel can be substituted for natural gas used in a gas turbine which enables solid fuel to be used for the entire cycle. For reasons of corrosion by sulfur and chlorine containing substances, the temperature of the fluid heated by this boiler must be limited to keep the temperature of the exchanger tubes below a threshold value above which rapid corrosion occurs. This waste heat boiler is currently of the subcritical type with one pressure level. This metal temperature limitation degrades the return in terms of the sensible heat content of the gas from the gasifier. For this reason gasification in a carbon conversion stage with fused ash has limitations in terms of cycle overall efficiency.
Gasification with carbon conversion in two stages appears to be more beneficial as there may then be practised in a fluidized bed in situ capture of sulfur by limestone to form calcium sulfide, which reduces the concentration of corrosive sulfur-containing substances in the waste heat boiler on the output side of the gasifier. The carbon-containing residue from the gasification process can then be burned in the circulating fluidized bed boiler.
The present invention proposes a heat recovery method which can significantly increase the overall efficiency of the cycle.